How can I create one large array across 3 controllers

You will control this through the "Disk Manager"

I'm assuming 2012 R2, Sever Manager / Tools / Computer Management / Disk Management

So just create my arrays on each controller, boot the server then use Disk Manager to make one large logical disk?

doing something like this implies software raid on top of hardware raid which makes no sense; should use one or the other

if you had 6 drives connected to each of the 3 controllers then used storage spaces (or disk management) to create a very large drive that spans across all of them, if one of those controllers fails bringing down a third of the storage, you will have serious issues overall

what controller do you have?  looking around i see it comes with the P420 which supports up to 60 drives (depending on the server chassis) so if that is what you have then why 3 controllers?  don't quite understand the logic here

There aren't enough connectors on the controller to connect three backplanes to it Seth, there are optional backplanes that have SAS expanders on them so one controller can handle all three but a bit late now since they have the higher performing 3 controller option.

Software striping or concatenation is really the only option if they need to have all the VMs on a single logical drive although IMHO it's better to just have more than one datastore / drive letter and spread the VMs about over them.

Thanks, it took a few reboots and disk scans to get the changes to show up but I have what I was looking for.

Did you use software striping to get it to show up as one big volume under Windows?

That is the way it is at the moment. However, I will probably redo it as RAID 10

Software RAID 0 is fine since you can use hardware RAID 10 on each controller for disk redundancy and performance. In effect you'll have RAID 10+0. I'd still rather see 3 separate volumes with VMs spread over them.